Ref [1]} gives greater insight into the reasons why all processes that we have so far been able to implement are causal: in fact, any process made up of localised events which is implemented in a fixed acyclic background spacetime in a way which satisfies relativistic causality (i.e. no superluminal signalling) can be rewritten using a `fine-graining' process which splits individual laboratories into several distinct laboratories in order to produce a fixed acyclic causal structure. This makes sense, since after all the acyclic background spacetime necessarily imposes a global strict partial order on any processes performed in that spacetime, if those processes are required to respect relativistic causality. In particular, even if the process involves quantum systems in a superposition of different causal histories, relativistic causality entails that `an agent has the potential to intervene at any of the spacetime locations to verify the probability of detecting the particle there ... (and) if agents choose to perform such interventions, this will not enable them to signal outside the future of the spacetime,' and it is this requirement which ensures that these processes can always be rewritten with a fixed causal order. Ref [1]} therefore argues that no experiment in a well-defined spacetime can demonstrate `indefinite causal order' in a strong sense - in their view, the only way to exhibit genuine `indefinite causal order' would be to do some kind of quantum gravity experiment where the spacetime background is no longer fixed. However, this still leaves open questions about how to interpret processes performed with a fixed background which at least superficially seem to involve superpositions of causal orders; we will return to this question in section .
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